This invention relates to optical communications systems and, more particularly, a pump control system for optical amplifiers for use in such systems.
Large capacity optical transmission systems typically combine high speed signals on a signal fiber by means of Wavelength Division Multiplexing (WDM) to fill the available bandwidth. In these WDM optical transmission systems, in general, rare-earth doped fiber optical amplifiers (such as Erbium or Erbium-Ytterbium doped) are used to compensate for the fiber link and splitting losses. Such amplifiers are provided with laser pump light to cause the optical amplification.
As the number of channels increases, the maximum output power requirements increase. There are various regulations concerning the maximum allowable power outputs for different classes of amplifier, and these different classes of amplifier require different safety measures. There is therefore a need to ensure that maximum power requirements are satisfied so that a particular amplifier can achieve a given classification.
Automatic gain control (AGC) schemes are well known, which maintain a constant gain, regardless of the number of channels present at the input. This overcomes the problem of gain transients, which occur when channels are added or dropped either due to network reconfiguration or failures. Adding channels can depress the power of the present channels below the receiver sensitivity. Dropping channels can give rise to error events in the surviving channels because the power of the surviving channels can surpass the thresholds for non-linear effects. The error bursts in the surviving or present channels as a result of these power transients are unacceptable to service providers. Various other factors also give rise to gain modulation, causing non-uniform amplifier gain.
Automatic gain control requires rapid gain control to respond to channel adding and dropping at the input, without giving rise to large or prolonged gain transient effects. Automatic gain control (AGC) is typically in the form of optoelectronic or all optical feedback loops for controlling the laser pump source to provide a required change in amplifier pumping. AGC schemes may use feedforward or feedback loops, or a combination of these, in order to derive control signals from measures of input and output powers so as to increase the amplifier pump power when more output power is required.
It is also known to provide a series of so-called concatenated amplifier stages within a single optical amplifier. Each stage has its own associated doped fiber section and pump sources. For each individual stage, the noise performance is improved for higher power operation. With this in mind, it has been recognised that the first stage within such an amplifier should be operated at the highest possible power, so that the noise introduced by the first stage, and which is amplified by subsequent stages, is kept to a minimum. Such an arrangement provides that the first stage of the optical amplifier always contributes to the output power of the amplifier.
According to the invention, there is provided an optical amplifier comprising:
a doped fiber and a pump source for providing pump light to the fiber;
a power measurement circuit for measuring the output power of the amplifier; and
a disable circuit for disabling the pump source in response to the output power of an output pulse exceeding a predetermined limit.
The invention provides an amplifier in which a disable function is provided, so that power surges can be avoided. Preferably, the predetermined limit takes into account the duration of the output pulse, so that the disable circuit can be tailored to pulse power versus pulse duration standardised limits.
For example, the disable circuit may implement a desired relationship between pulse duration and maximum pulse power, wherein the maximum pulse power is, for all pulse durations, below a predetermined pulse power classification. This classification may comprise the 3A optical amplifier classification.
The amplifier may comprise at least first and second amplifier stages, wherein the disable circuit is for disabling the pump source of at least the first amplifier stage. For all levels of gain of the amplifier, the pump source of the first amplifier stage may be operated to provide signal gain. A differential pump drive scheme can reduce noise in the amplifier, and in this way, linking the disable function to the first amplifier stage will always provide a reduction in output power.
The power measurement circuit is preferably also for measuring the input power of the amplifier; and a driver circuit is provided for generating pump control signals for controlling the pump source of each amplifier stage in dependence on the input and output power in order to maintain a substantially constant gain.
Preferably, the disable circuit comprises an integrator for integrating the output power pulse and a comparator for comparing the integrated output power pulse with a threshold level.
The invention also provides an optical communications system comprising a transmitter and a receiver connected together by an optical fiber link, the optical fiber link including at least one amplifier according to the invention.
The invention also provides a method of controlling an optical amplifier comprising a doped fiber, a pump source for providing pump light to the fiber and a power measurement circuit for measuring the output power of the amplifier, the method comprising disabling the pump source in response to the output power of an output pulse exceeding a predetermined limit.